Lung ventilator

ABSTRACT

A lung ventilator is provided with means for controlling the magnitude of a variable of the flow of respiratory gas in the inspiration line or in the expiration line or in both of them and means for automatically modulating said control means in response to the instantaneous magnitude of said variable of said flow thereby maintaining a desired pattern of said magnitude of the flow.

United States Patent [1 1 Jonsson et a1.

[ June 26, 1973 LUNG VENTILATOR 3,556,098 1/1971 Kanwisher 128/142 3,524,444 3/1966 131111111 128/142 1 lnvemrs= 1" Stavgata 3,316,902 5/1967 Winchel.... l28/145.5 Lemma; Sven lngelsledt, 3,523,527 8/1970 Foster 128/1456 Nehrmansvag n 2 n Sven 3,033,195 5/1962 Gilroy 128/1458 Gunnar Olsson, Lillhersbyvagen 2,830,583 4/1958 Finney 128/142 10A, Sollentuna, all of Sweden 1 Filedl 1971 Primary Examiner-Richard A. Gaudet l: 117 89 Assistant Examiner-G. F. Dunne [21] Appl No Att0rneyPierce, Scheffler & Parker I Related U.S. Application Data [63] Continuation-impart of Ser. No. 749,205, July 31, 1968, abandoned. [52] Cl 128/145'6 137/82 137/102 A lung ventilator is provided with means for controlling [51] 1111. C1 A61!!! 16/00 the magnitude of a i l of the flow of respiratory [58] P181131 01 Search 128/145, 145.5, 145.6, gas in the inspiration line or in the expiration line or in 128/145-81 188; 137/102 both ofthem and means for automatically modulating 88; 73/198 279 said control means in response to the instantaneous magnitude of said variable of said flow thereby main- 1 References C'ted taining a desired pattern of said magnitude of the flow.

UNITED STATES PATENTS 3,252,458 5/1966 Krasberg 128/147 14 Claims, 10 Drawing Figures to and from patient CONSTANT PRESSURE INSPIRATION GAS R METE E RATION G O SERVO UNIT MONITOR SERVO UNIT REFERENCE VOLTAGE GENERATOR PAIENIEMunzs l975 sum 1 0 6 to and from patient CONSTANT PRESSURE INSPIRATION G ARAME ER EXPlRARATlON GAS SOURCE 3 sERvo UNIT) MONITOR SERVO UNIT) 1 REFERENCE VOLTAGE g- GENERATOR 2 FLOW METER FLOW CONTROL UNIT UNIT I V I ZERO ERRoR SIGNAL H FT I S I 9 7 CALCULATOR l I LINEARIZER INTEGRATOR from I -0 3 PATENTEU JUN 26 I973 SHEET? [1F 6 F lg. 3

FLOW METER FLOW CONTROL l UNIT mm H I ,fi 12 14 ,17

NON-RETURN VALVE ZERO sHlFT ERROR SIGNAL 15 100m? 13 ""CALCULATOR \LINEARIZER from 3 F lg.

to 2 to 3 from 3 t 1.

Y A A SIGNAL LOGIC GENERATOR cmcun j I \I i! I j ICOUNTER COUNTER COUNTER I (CLOCK PULSE GENERATOR Pmmenmzs 19:5 3141; 20s

SHEET 3 OF 6 AVERAGE FLOW MEAN VALUE INDICATOR INTEGRATOR) from 4 25 31 30 PREssuRE/ A INDICATOR HIGH PRESSURE ALARM or:

LFLOW [MIT 26 27 A ARM 32 28 29 ACOUSTIC ACOUSTIC ALARM ALARM DISCONNECT to 3 from 3 PAIENIEBmas 191s 3.741.208

SHEEI 5 [1F 6 50 6-1 If I I I0 FLOW V I METER UNIT L I I l FLOW I .1 I

- ,ZEROIZER %%f 51 1/ FOR FLOW METER '1 DURING GAS CUT-OFF INTERVAL 52 9--1 I L 35 3: J F lg. 8

61 I FLOW REGULATOR 64 62 PATIENT'S LUNGS 67 SWITCHING SOLENOID 63 65 f J FLOW REGULATOR I PRESSURE SENSING 7 1 TRANSDUCER RESPIRATION CYCLE 69 PRODUCER SWITCHING SOLENOID LUNG VENTILATOR This application is a continuation-in-part of copending U.S. Pat. application Ser. No. 749,205, filed July 31, 1968 now abandoned.

The present invention relates to lung ventilators.

Prior art lung ventilators of the type providing automatic positive over-pressure ventilation of the lungs operate according to different modes. In pressure controlled lung ventilators the pressure of the respiratory gas is varied in a predetermined manner in a tube or hose, connected to the respiratory passages of the patient. The gas volume supplied to the patient at each breath in this type of lung ventilator will vary in dependence of the pressure variations as well as the condition of the respiratory passages and the lungs of the patient. Therefore, the ventilation cannot be directly preset in the lung ventilator, and this is a substantial disadvantage extensively limiting the utility of this type of lung ventilator in advanced respiratory treatment.

Volume controlled lung ventilators operate to supply a predetermined adjustable volume of respiratory gas to the patient at each breath the ventilation thereby being determined substantially by the adjustment of the lung ventilator.

Prior art volume controlled lung ventilators are of two different types. In one type bellows or other container is filled during the expiration phase to predetermined degree with respiratory gas such gas being supplied to the air passages and lungs of the patient during the inspiration phase. These lung ventilators are bulky, noisy, mechanically complicated and generally difficult to clean and sterilize. Moreover, they generally do not allow patient responsive control, i.e., that the patient starts the supply of respiratory gas from the lung ventilator by an effort to perform an inspiration, since the respiratory cycle is fixed by the mechanical components of the lung ventilator. For the same reason it is not possible to control, without difficulty, the ratio of the periods for inspiration and expiration. Further, the lung ventilators of this type provide large compressible volumes, which means that whenthe pressure in the air passages of the patient rises part of the gas supplied by the bellows or other container will return to increase the pressure in the compressible volumes. This involves difficulties to determine and to maintain the gas volume supplied to thepatient. Finally, volume controlled lung ventilatorsof the type described take air from the surrounding space to be used as respiratory gas, dust and bacteria entrained in the air being supplied to the respiratory passages of the patient which involves risk for infections since filters provided in the lung ventilator generally do not completely or sufficiently clean the air supplied as respiratory gas.

The second type of volume controlled lung ventilator operates according to the high impedance mode wherein the respiratory gas is exposed to a very high constant pressure and a large constant flow resistance is imposed to the gas when allowed to flow to the patient during the inspiration phase. Since the pressure variations in the respiratory passages of the patient are small compared to the high primary pressure the gas flow is substantially constant during the inspiration phase. Thus, the gas volume during each breath will be governed by the rate of flow and by the period of each inspiration. A disadvantage of this type of lung ventilator is the difficulty to maintain the high primary pressure at a constant level when large variations of the output flow rate occur. This necessitates a very high capacity of the high pressure apparatus. Moreover, this type of lung ventilator cannot be used when supplying certain anaesthetic agents which may cause explosions at the high pressure involved. Further, such pressure necessitates a heavy and complicated control equipment to be used.

In the above lung ventilators the respiration pattern is defined by the structure of the lung ventilator in question without any possibility of changing the respiration pattern. As patients with obstructive o'r restrictive pulmonary diseases and with circulatory disturbance have different optimal insufflation patterns, said lung ventilators are insufficient for the ventilation of several patients.

In order to detect a leakage in the patient circuit the prior lung ventilators have required the use of a gas I meter by means of which the gas volume expired during one minute has intermittently been measured. This method is laborious and often delays the detectionof such a leakage.

The object of the invention is to provide a lung ventilator in which said disadvantages have been obviated.

In accordance with the invention there is provided a lung ventilator comprising a respiratory circuit having an inspiration connection line and an expiration connection line, means for generating a flow of respiratory gas to said respiratory circuit through said inspiration connection line and means for discharging a flow of re spiratory'gas from said respiratory circuit through said expiration connection line, means for controlling the magnitude of a variable of said flow of respiratory gas through at least one of said inspiration and expiration lines, and means for automatically modulating said control means in response to the instantaneous magnitude of said variable of said flow thereby maintaining a desired pattern of said magnitude of said flow.

Two different embodiments of the lung ventilator according to the invention will be described with reference to the accompanying drawings in which FIG. 1 is a block diagram of a first embodiment of a lung ventilator.

FIG. 2 is a block diagram of an inspiration servo unit of the lung ventilator of FIG. 1. I

FIG. 3 is a block diagram of an expiration servo unit of the lung ventilator of FIG. 1.

FIG. 4 is a block diagram of a reference voltage generator of the lung ventilator of FIG. 1.

FIG. 5 is a block diagram of a monitoring unit of the lung ventilator of FIG. 1.

FIG. 6 is a perspective view of a flow meter of the lung ventilator of FIG. 1.

FIG. 7 is a sectional view of a servo valve of the lung ventilator of FIG. 1.

FIG. 8 is a schematic diagram of a zero-shift compensating device of the lung ventilator of claim 1.

FIG. 9 is a diagrammatic, partially sectional view of a second embodiment of the lung ventilator according to the invention having a flow regulator in the inspira-' tion connection line as well as the expiration connection line of the respiration circuit; and

FIG. 10 is a block diagram of the components of an electronic unit and a transducer forming part of the lung ventilator shown in FIG. 9, together with associated solenoids of the flow regulators and a supplementary systemv providing a sigh feature in the lung ventilator.

Referring to FIG. 1 respiration gas is supplied to the The flow meter unit 6 does not give a linear output signal and this non-linear signal is fed to a linearizing and setting unit where the signal is linearized in a respirator or lung ventilator from some type of pressure source and is supplied via non-return valves and sterilizing filters to a device 1 which has the purpose of giving the working pressure of the lung ventilator a constant value. The device 1 has as its principle part a bellows to which the respiration gas is supplied and which is exposed to a compressing force which is constant within a predetermined working range independently of the filling degree of the bellows.'From the pressure source the bellows is supplied with respiration gas through an on-demand mechanism automatically maintaining the filling degree of the bellows to about the half volume thereofv Thus, the bellows acts both as a reservoir and as a pressure reducting valve. The bellows is provided with several inlets for allowing the mixing of gases of different kinds before the gasesare supplied to the patient.

The gas flow from the device 1 to the patient is controlled by means of a servo unit 2 which continuously keeps the flow to the patient set in response to an electric signal which is supplied to the servo unit. 2 from a reference voltage generator 3. Also the gas flow from the patient may be controlled by means of a servo unit 4 which in response to an electric signal from the reference voltage generator 3 determines the gas flow from the patient. Thus, the inspiration servo unit 2 and the expiration servo unit 4 operate'alternatingly. The lung ventilator also has a monitoring unit 5 which monitors the pressure in the patient circuit, the expired volume of respiration gas and/or other desired variables. The monitoring unit 5 generates output signals in dependence on. the sensed variables in order to actuate, if desirable, the reference voltage generator in such a way that its output signal to the servo unit 2 and/or the servo unit 4 is corrected in the desired manneigor in order to give an alarm if the desired ventilation course cannot be maintained, eg due to leakage in the connections to the patient.

The servo unit 2, which continuously keeps the flow to the patient set in dependence on an electric signal which is supplied to the servo unit 2 from the reference voltage generator 3 will be described in the following with reference to FIG. 2. The servo unit acts in principle in such a way that a flow-dependent signal generated by a flow meter unit 6 consisting of a flow meter and an amplifier is supplied to an error signal calculator 7 in which the signal is compared with the signal from the reference voltage generator and which in dependence hereon generates an error signal which is supplied to a flow control unit 8 in order to adjust said unit in such a way that the gas flow to-the patient desired at any moment and defined by the reference signal is obtained. The servo unit 2 further contains a device 9 which is adapted to compensate for zero shift, if any, in the flow meter unit 6. This is achieved by the device 9 by zeroizing the flow meter unit 6 in a manner described in greater detail in the following each time it can be reliably established that the flow through the flow meter unit is zero, which is the case when the flow control unit 8 is closed. Thus, the device 9 zeroizes the flow meter unit 6 every time when it receives a signal from the control unit which indicates that the control unit is closed.

diode filter in a conventional manner. The quantity of gas supplied to the patient, the so-called minute volume, may be controlled by means of the unit 10 by changing the amplification of the signal which is now linearly dependent on the flow. The minute ventilation may in that case be set between 0.5 and 30 liters per minute. The linearly flow-dependent signal is supplied, as previously mentioned, to the error signal calculator in order to control the flow control unit in such a way that the desired flow is maintained at any moment.

In order to achieve that the average value of the flow becomes correct in spite of the momentaneous error in the desired flow which is due to the inherent inertia of the system, the servo system 2 contains an integrator 1 l which is supplied with the linearly flow-dependent si'gnalifrom the linearizing and setting unit 10 and is supplied with the signal from the reference voltage generator 3. In the integrator 11 the error or deviation between the desired and the actual flow is integrated by integration of the difference between the signal received via the linearizing and the setting unit from the flow meter unit 6 and the said referencesignal from the reference voltage generator, an output signal generated by the integrator in dependence hereon being supplied to the error signal calculator 7 in order to actuate the output signal thereof in such a way that the remaining error will be 0. If the patient coughs e.g., thus building up such a large counter pressure against inspiration that the desired flow cannot be maintained during an inspiration in spite of the valve being open to its maximum, a signal is generated by the integrator which achieves a compensation for this during the subsequent breaths. As a result of this the average flow becomes correct in spite of the fact that a too small volume of respiration gas during a ventilation cycle has been supplied to the patient.

The expiration servo unit 4 which is illustrated in FIG. 3 as a block diagram, may be used in the same manner as the servo unit 2 in order to keep the gas flow from the patient set at any moment in dependence on an electric signal which is supplied to the expiration servo unit 4 from the reference voltage generator3. Like the inspiration servo unit 2, the expiration servo unit 4 has a flow meter unit '12 which generates a signal which is dependent on the gas flow from the patient. The flow meter unit 12 feeds its flow-dependent signal to an error signal calculator 13 in which the signal is compared to the signal from the reference voltage gen- 1 erator and which generates, in dependence thereon, an error signal which is supplied to a flow control unit 14 in order to set it in such a way that the desired gas flow 7 from the patient is maintained at any moment. Like the inspiration servo unit 2,, the expiration servo unit 4 has a device 15 for compensating zero shift, if any, in the flow meter unit and a linearizing unit 16 in which the signal from the flow m eter unit 12 is linearized in a conventional manner before it is fed onto the error signal calculator 13. The linearized signal from the linearizing unit 16 is also fed to the monitoring unit 5 of the lung ventilator, where the signal is used in a manner described in greater detail in connection with a description of the monitoring unit 5 in order to indicate the average flow of the respiration gas leaving the patient.

For a reason which will be indicated in greater detail in the following description of the monitoring unit 5 there is also provided a non-return valve 17 after the flow regulator 14 of the expiration servo unit 4.

Even if it should consequently be possible to use the expiration servo unit 4 in order to determine the flow of the expired gas in basically the same way as the inspiration servo unit 2 determines the flow of the inspiration gas, i.e., in such a way that the expiration servo unit at any moment determines the flow in response to a signal from the reference voltage generator, such a control of the expiration flow is as a rule not necessary. Instead it is sufficient to let the expiration servo unit 4 operate with the control valve completely open during the whole expiration phase.

However, during free expiration by patients having certain types of breathing difficulties there may arise a suction action due to high air flow velocity which causes the walls of the air passages to stick to each other. This may be prevented by supplying a signal to the expiration servo unit 4 from the reference voltage generator 3, said signal indicating the maximum permitted flow during expiration and setting the flow control unit 14 accordingly.

The reference voltage generator 3 of the lung ventilator will be described in greater detail in the following with reference to the block diagram in FIG. 4. The reference voltage generator has a clock pulse generator 18,'such as an astable multivibrator. In the said clock pulse generator pulses are generated with a frequency which is one hundred times asgreat as the breathing frequency, and hence a breathing cycle will comprise 100 clock pulses. By setting the clock pulse frequency of the clock pulse generator it is consequently possible to determine the respiration frequency. The clock pulses are fed to and counted in a decade counter 19 which is connected in such a way that it will return to 0 after 100 pulses and will then resume its counting anew. On the decade counter 19 the length of the inspiration time may be selected by setting the number of pulses to which the inspiration time is to correspond, the counter generating a signal when the set number of pulses has been reached. It is possible to set the counter 19 so that the inspiration time will amount to 15-33 percent of the whole ventilation cycle. The signal which is supplied to the inspiration servo unit 2 from the reference voltage generator 3 is generated in a signal generator 20. To the signal generator clock pulses are fed from the clock pulse generator 18 together with a signal indicating that there is an inspiration phase, from the decade counter 19. In the signal generator 20 there is generated a voltage which is inversely proportional to the length of the inspiration phase through the length of the respiration cycle in a voltage divider which is ganged to the switch for selecting the length of the inspiration time. In the signal generator 20 this voltage is integrated with the clock pulses in such a way that the area of the reference signal becomes independent of the relation between the length of the inspiration phase and the length of the respiration cycle and that the area of the reference signal during the inspiration phase will vary with the clock pulse frequency so that the surface of all reference signals will be constant during a determined period of time. Preferably, the signal generator 20 is adjustable in such a way that the reference signal which is supplied to the inspiration servo unit will either correspond approximately to a semi-period of a sinus wave or is square in order to provide an inspiration gas flow of the corresponding shape. It will easily be realized that the signal generator may also be caused to generate other desired forms of flow, e.g. a decelerating flow during the inspiration phase. When the inspiration time is over, i.e., when the number of pulses corresponding to the inspiration time has been counted, the inspiration is interruptedand a binary counter 21 is switched on, which is adapted to determine the length of the pause time in the same way as when determining the inspiration time, i.e., in such a way that the pause time corresponds to a predetermined number of pulses. During the pause both the inspiration valve and the expiration valve are closed so that the respiration gas is retained in the lungs of the patient. The pause time can be selected to a value of between 0 and 20 percent of the'ventilation cycle. The expiration starts when a logic circuit 22 senses that there is neither inspiration phase nor pause phase, the circuit generating an expiration signal. The expiration will then continue during the number of pulses which remains after the number of pulses that corresponds to the expiration and pause before the decade counter has counted to one hundred. Thus the expiration time can be selected to a value of between 47 and percent of the ventilation cycle. in the described'use of clock pulses for controlling the respiration course the relation between the inspiration time, the pause time and the expiration time will be maintained independently of whether the respiration frequency is changed, which is a palpableadv'antage. v

The reference voltage generator has an additional counter 23 which is adapted to achieve a sigh function. Preferably, the counter 23 is of such a kind that it will bring about after counting respiration cycles that the pulse frequency of the clock pulse generator during the subsequent inspiration, i.e., during thesubsequent generation of the number of clock pulses corresponding to the inspiration will generate the clock pulses with a frequency amounting to only half or one third of the pulse frequency during the normal respiration course. The length of the sigh function may be selected and the sigh function may be switched on by means of a special switch. The result of this is that every hundred breath obtains twice or three times as large volume as normal, so that the lung is inflated in a manner corresponding to a sigh. The clock pulse generator 18 will also feed its clock pulses to the monitoring unit 3, where the clock pulses together with the said flow-indicating signal from the expiration servo unit 4 are utilized in a manner described later in order to bring about a mean value indication of the gas flow from the patient. The decade counter 19 for determining the inspiration time is connected with the monitoring unit in order to permit the inspiration to be effected or controlled in dependence of a condition sensed by the monitoring unit 3.

The monitoring unit 5 will be described in greater detail in the following with reference to FIG. 5. The monitoring unit 5 has a pressure meter 24 connected to the conduit between the inspiration servo unit 2 and the patient, said meter measuring the pressure of the gas supplied to the patient. The pressure meter 24, which also comprises an amplifier, generates an electric signal in dependence on the said pressure, and the said signal is fed to an indicating instrument 25 which indicates the said pressure directly. The pressure signal from the pressure meter 24 is also fed to two alarm devices, one

alarm device 26 for a lower limit value of the pressure and one alarm device 27 for an upper limit value of the pressure. In the alarm devices 26 and 27 the signal from the pressure meter is consequently compared with the set minimum and maximum values, respectively, of the gas pressure, the alarm devices 26 and 27 indicating when the pressure has decreased below the lower limit value or when the upper limit value has been exceeded. The alarm device 26 for the lower limit value has an optical device indicating that the pressure has decreased below the lower limit value. The alarm device 27 for the upper limit value also has an optical device for indicating that the upper limit value has been exceeded and is besides connected to an acoustic alarm device 28 giving a signal when the upper limit value is exceeded. The acoustic alarm device 28 is provided with a disconnecting means 29 in order to make it possible to disconnect the acoustic alarm during a short period, e.g. 2 minutes, during intentional, temporary interruptions in the ventilation, e.g. when the patient is being cared for.

The monitoring unit also has a mean value integrator 30 which as mentioned previously is supplied with a signal indicating the instantaneous flow from the expiration servo unit 4 and is supplied with clock pulses from the clock pulse generator 18 of the reference voltage generator 3. In that connection the clock pulses control the mean value integrator 30 in such a way that the time constant of the integrator is inversely proportional to the ventilation frequency. This is brought about by prolonging the clock pulses and making them open a field effect switch energizing the mean value integrator during the duration of the prolonged clock pulse. The mean value integrator 30 consequently operated during a respiration cycle during the same time independently of the length of the respiration cycle so that the time constant of the integrator, as mentioned, will be inversely proportional to the respiration frequency. As a result of this, the integrator will always react with suitable speed. The output signal from the mean value integrator 30 is fed on one hand to an indicating instrument 31 directly showing on an operating panel the average flow in e.g. liters per minute and on the other is fed to a limit value alarm device 32 in which the signal from the mean value integrator 30 is compared to the maximum and minimum values of the minute volume. If the minute volume determined by the mean value integrator exceeds the maximum minute volume or is less than the minimum minute volume an optical alarm is given on the limit value alarm device 32 and a signal is fed on to the acoustic alarm device 28 which generates an acoustic signal when the said deviation from the set minute volume occurs. I

When the pressure drops below the lower limit value the signal from the alarm device 26 for the lower limit value of the pressure is fed to the reference voltage generator 3 and more particularly to its decade counter 19 in order to zeroize said counter and cause the decade counter to start a new respiration cycle at once. The signal from the alarm device 26 is fed, when the upper limit value of the pressure is exceeded to the decade counter 19 of the reference voltage generator in order to interrupt the respiration cycle and start expiration immediately.

The lower alarm limit of the pressure may also be utilized for letting the patient control the lung ventilator.

I If the patient during the expiration course makes an attempt to inspire a subpressure is generated in the patient circuit, whereby the pressure drops below the lower alarm limit and a new inspiration cycle is begun in accordance with the course described above. The non-return valve 17 described in connection with the expiration servo unit 4 will be closed when the patient makes spontaneous attempts at inspiration so that the necessary subpressure may be generated in the circuit.

As mentioned the inspiration and expiration servo units 2 and 4, respectively, are provided each with one flow meter which is preferably of the kind shown in FIG. 6. The respiration gas flows to two parallel passages 33 and 34 into the flow meter. The major passage 33, through which the main portion of the gas flows, has a fine-mesh netting 35 (450 mesh per inch). The netting 35 causes a pressure drop, which is essentially directly proportionally to the flow through the passage. In the minor passage 34 there is disposed a round thin disc 36 which is carriedat one end by a rigid thread 37. The disc 36 is disposed at right angles through the direction of flow of the gas. The rigid thread 37 rests against a pressure sensitive member 38 which is preferably constituted by a thin silicon plate. At either sides of the silicon plate 38 there are two resistors diffused into the device and said resistors are connected in a bridge connection. The gas flowing through the minor passage 34 exerts a pressure on the silicon plate 38 via the rigid thread 37 in such a way that said plate is bent, the resistance of one resistor increasing while the resistance of the other resistor decreases. As a consequence of this the flow meter will give an output signal which is in a predetermined relation to the flow. As mentioned, the output signal is then linearized in the unit 10 in the inspiration servo unit 2 and in the unit 16 in the expiration servo unit 4. In order to prevent condensate from forming when the respiration gas is moistened it is preferred to heat the flow meter by means of a resistor cast into it. The great advantages of the flow meter described is that it can measure frequencies as rapid as 200 c.p.s. and also that it is autoclaveable.

In addition, each of the inspiration servo unit and the expiration servo unit has a servo valve which is part of the respective control unit and which is preferably of the kind shown in FIG. 7 The servo valve consists of a step motor 40 the axle 41 of which is provided with a metal plate 42. In the plate 42 there is shaped a body provided with a cam surface 43 contacted by a cam follower 44 fixed to a lever 45. The leaver 45 is moveably mounted on a shaft 46 and rests at the edge of one of its ends against a hose 47 of silicon rubber through which the respiration gas flows to and from the patient, the hose being connected into the conduits leading to and from the patient. When the step motor operates the hose 47 will be squeezed or opened in dependence on the direction of rotation of the motor. A position indicator (not shown), which consists of a lamp and a photo cell senses the position of the valve and gives a signal when the valve is completely closed. This signal is utilized in a manner described later in order to bring about zeroizing of the flow meter. The cam surface 43 of the valve is preferably designed in such a way that the flow within the whole flow region during normal control will be changed 10 percent from the previous value for each displacement of one step by the step motor. Thus, a logarithmic control of the flow takes place, which also entains that the loop amplification in the servo system becomes constant within the whole control region. This is required in order that stability in the servo system may be obtained. This gives a resolution of i percent but this value will be improved by the motor reciprocating during control above the rate value. The times during which the flow is too large or too small are controlled in such a way that the average flow becomes correct. The step motor is driven at 500 steps per second and from a completely open to a completely closed valve there is required a time of about l/l0 second. An advantage in the described regulator is that the parts thereof contacting the respiration gas are autoclaveable.

The inspiration servo unit 2 and the expiration servo unit 4 contain, as mentioned, a device 9 and 15, respectively, which is adapted to compensate for zero-shift, if any, in the flow meter unit 6 and 12, respectively, consisting of a flow meter 48 and an amplifier 49. The said device is shown in FIG. 8 and comprises a capacitor 50, a field effect switch 51 and a sensing means 52 disposed in the servo valve which has previously been mentioned and is adapted to indicate when the servo valve is closed and the flow through the flow meter is consequently 0. When the flow is 0, which is the case during the expiration phase, the servo valve is consequently closed, and in that case the field effect switch 51 is short-circuited in such a way that the capacitor 50 is charged, the input voltage to the amplifier 49 becoming 0. During the inspiration phase, when the field effect switch 51 is consequently open, there can consequently not take place any change worth mentioning in the charge of the capacitor 50 due to the fact that the subsequent amplifier has a very high input impedance.

in the respirator according to the invention there are unique possibilities of determining the resistance and compliance of the patient. For this purpose the electric output signals from the flow meter units and the pressure meter of the lung ventilator are utilized. The signals are supplied to a recorder and the curves obtained in that connection may be used for determining the said resistance and compliance.

The described servo system of the lung ventilator according to the invention is completely flow-dependent but it is also possible to provide the lung ventilator with a servo system operating in dependence on the pressure or a system where control is achieved by means of the product of the pressure and the flow.

By connecting an underpressure source to the expiration passage the emptying of the lungs of certain patients may be facilitated.

When using expensive narcoses gases the lung ventilator may operate with a circle system, i.e., the expired gas is fed back to the inspiration side via a compressor after being supplied with a substitute for the consumed parts of the gas and removing for instance carbon dioxide.

Referring to FIG. 9 in the drawing the lung ventilator shown therein comprises an over-pressure source 61 such as a motor driven pump or a reservoir for respiratory gas supplying such gas to the lung ventilator under a pressure preferably in agreement with the maximum pressure compatible with the respiratory passages and lungs of the patient. Source 61 is connected by a flexible tube or hose 62 for supplying respiratory gas to the patient whose respiratory system including the respiratory passages and the lungs is diagrammatically indicated in FIG. 9 by a volume 63. In tube 62 there is provided a flow regulator 64 controlling the flow of respiratory gas to patient 63 so as to keep the flow rate at constant level during the inspiration period, pressure changes during said period on each side of flow regulator 64 having no influence on the constant flow rate. Flow regulator 64 is preferably of the type described in US. Pat. No. 3,502,100 comprising a valve member 64, actuated by a flow rate responsive differential pressure appearing over a restriction 64" in tube 62 having the flow regulator incorporated therein, in order to throttle the flow in the passage for the respiratory gas through the flow regulator if said differential pressure exceeds a predetermined value, said flow rate thereby being maintained at the level providing said differential pressure. Further details regarding the construction of flow regulator 64 may be had from the Patent-referred to above.

However, in general, it can be said that should there be an increase in the rate of flow from the overpressure source 61 through conduit 62 to the inlet side of regulator valve 64, the pressure difference between this conduit and the cylinder 640 within which the valve piston 64' operates, communicated to the cylinder via the pipe connection 64b extending from a junction with conduit 62 upstream from the restriction 64", will increase. When the pressure difference reaches a certain value depending on the size of the restriction 64", the valve piston 64 will be raised and flow through the opening 640 is restricted to such a degree that the flow will be exactly of such a size that the pressure difference will balance the gravity of valve piston 64' evenly whether or not the pressure of the fluid in tube 62 at the inlet side of regulator 64 increases further. Thus, the resistance to flow of the fluid through the regulator 64 is automatically modulated in response to the instantaneous value of the flow rate.

A flexible tube or hose 65 is connected to the respiration system of the patient 63 and also therein is incorporated aflow regulator 66 with a valve member 66' controlling the flow rate in tube 65 during the expiration period so as to prevent said flow rate to exceed a predetermined level. Flow regulator 66 is of the same type as and may be identical with flow regulator 64 and is responsive to the differential pressure over a restriction 66" in tube 65.

The switching between the inspiration and expiration phases is controlled by solenoids 67 and 68 incorporated into flow regulators 64 and 66 in the manner described in the above mentioned Patent. When energized solenoid 67 or 68 actuates the valve member in the associated flow regulator the flow through the flow regulator thereby being cut off. As will be understood flow regulator 64 is' open and flow regulator 66 is closed during the inspiration period the opposite conditions prevailing during the expiration period. A respiration pause may be introduced by closing both flow regulators simultaneously.

Solenoids 67 and 68 are energized and de-energized in response to an electronic and preferably transistorized unit 69 in order to achieve a desired respiration frequency and a desired ratio between inspiration and expiration periods and also a desired pause, if any, between such periods. Associated with unit 69 is a transducer 70 connected to the respiration circuit and responsive to the pressure therein to produce electrical signals supplied to unit 69 and affecting the control of solenoids 67 and 68 provided by such unit. Unit 69 and transducer 70 are shown in more detail in FIG. 10 and now will be described with reference tothat figure.

Referring to FIG. electronic unit 69 comprises an astable multivibrator 80 which controls the-respiration frequency, i.e., the number of respirations per minute. This multivibrator incorporates an RC-circuit in the conventional manner and an element in such RC- circuit is adjustable to vary the respiration frequency. Multivibrator 80 is of a type which may be triggered to be momentarily returned from one of its quasistable states to the other of said states although the period for the multivibrator to be in said one state as determined by the RC-circuit has not lapsed. Multivibrator 80 produces an output trigger pulse which is supplied by a line 81 to one of two trigger inputs of a monostable or oneshot multivibrator 82 said latter multivibrator being arranged to supply an output pulse in a line 83 of a predetermined duration when triggered to the quasistable state by the output trigger pulse from multivibrator 80. The duration of such pulse is determined by a conventional RC-circuit in multivibrator 82 an element of such circuit being adjustable to determine the length of the pulse from multivibrator 82. Line 83 is connected to solenoid 67 in inspiration flow regulator 64 and the output pulse of multivibrator 82 controls the energization of solenoid 67. If there is maintained in line 83 a predetermined potential energizing solenoid 67 to maintain valve member 64' associated therewith in the closed position then multivibrator 82 produces in line 83, when triggered, a pulse of the opposite polarity in order to de-energize solenoid 67. Thus, this solenoid will be de-energized during the output pulse from multivibrator 82 and flow regulator 64 will be kept open during this pulse and will operate to maintain the flow rate through the inspiration tube at a constant predetermined level. It will be seen that multivibrator 82 governs the period of inspiration.

Multivibrator 62 is arranged to produce in dependence of the rear or lagging edge of the pulse supplied to line 83 a trigger pulse in a line 84 connecting multivibrator 82 to a further monostable multivibrator 85 said trigger pulse being used to trigger multivibrator 85. Thus, when the output pulse from multivibrator 82 in line 83 disappeares multivibrator 85 is triggered to its quasistable state and a pulse is supplied to a line 86 connecting multivibrator 85 to one of the inputs of an AND gate 87 another input thereof being connected by a line 88 to line 83. The output of gate 87 is connected by a line 89 to solenoid 68 associated with expiration flow regulator 66. Gate 87 is arranged to be in open state when both multivibrators 82 and 85 are in their stable state. When in its open state gate 87 supplies a pulse in line 89. In line 89 there is normally maintained a potential sufficient to energize solenoid 68, and the pulse supplied by gate 87 is of opposite polarity to deenergize solenoid 68. When gate 87 receives a pulse from multivibrator 85 through line 86 as a consequence of said multivibrator being triggered to its quasistable state by the trigger pulse from multivibrator 82 no pulse is supplied by gate 87 to line 89. Thus, both solenoids 67 and 68 will be energized when multivibrator 85 is in its quasistable state. The duration of this condition is determined by an RC-circuit in multivibrator 85 and this duration determines in turn a pause during which'both flow regulators 64 and 66 are positively closed by solenoid 67 and 68, respectively. An element in the RC-circuit of multivibrator 85 is adjustable to adjust the duration of the pause introduced in the inspiration cycle by multivibrator 85.

When multivibrator 85 returns to its stable position after the period established by the RC-circuit therein has lapsed no pulse will be supplied to gate 87 through line 86. Thus, a pulse will be supplied by gate 87 through line 89 in order to de-energize solenoid 68 starting the expiration phase. This phase 66 will continue until a new inspiration trigger pulse is supplied by multivibrator in line 82 bringing multivibrator to its quasistable state, since the conditions for the gate to be in its open state then are no longer satisfied.

The operation of electronic unit 69 and associated solenoids 67 and 68 would be clear from the description of the construction thereof. However, befor describing further components connected to unit 69 the function thereof will be briefly described.

The breathing frequency is controlled by astable multivibrator 80 which triggers multivibrator 82 by its output trigger pulse. When triggered multivibrator 82 changes to its quasistable state, and in this position solenoid 67 is deenergized. The inspiration period is started. When the time interval set in multivibrator 82 has lapsed this multivibrator returns to its stable state and solenoid 67 is energized to close flow regulator 64. At the same time a trigger pulse is supplied to multivibrator 85 which switches to its quasi-stable state to start a pause both solenoids 67 and 68 being energized by multivibrator 82 and gate 87, respectively. After the time interval preset in multivibrator 85 has lapsed this multivibrator returns to its stable state. The conditions for gate 87 to open thereby are satisfied (both multivibrators 82 and 85 are in the stable state) and solenoid 68 is deenergized. Thereby, the expiration period is started and will continue until a new pulse for trigging multivibrator 82 is supplied by multivibrator 80. The conditions for gate 87 to be open thereby are no longer satisfied; solenoid 68 will be deenergized to close the expiration period.

Thereafter the operation cycle as described will be repeated.

As mentioned above the period of the complete respiration cycle is controlled by an RC-element in multivibrator 80, the period of inspiration in relation to the period of the complete cycle is controlled by an RC- element in multivibrator 82, and the period of pause in relation to the complete cycle is controlled by an RC- element in multivibrator 85. A second RC-element of each of multivibrators 82 and 85 may be ganged to the adjustable RC-element in multivibrator 80 to be adjusted simultaneously therewith. In this way the ratio between the inspiration period and the complete cycle period and the ratio between the pause period and the complete cycle period, respectively, is adjusted by the first RC-element in each of these multivibrators. When the respiration frequency is changed by adjusting the RC-element of multivibrator 80, the second RC- elements in multivibrators 82 and 85 are adjusted to maintain the ratios set therein by said first RC-element. As an example the lungs of a patient are to-be ventilated by a flow of i0 liters/min. and at a frequency of 20 respiration cycles/min. The frequency is set in multi vibrator 80 and the inspiration period is set in multivibrator 82 and is chosen to be e.g. rt; of the complete cycle period. The inspiration flow rate is adjusted to 3 X 10 30 liters/min. by adjusting flow regulator 64. Thus, the patient will be supplied with 30 liters/min. of respiratory gas during it of the complete cycle period,

i.e., l liters/min. The expiration flow may be adjusted analogically if desired.

Referring to FIG. transducer 70 comprises a pressure sensitive apparatus 90 providing an electric signal which is proportional to the pressure in the respiration system. This apparatus may be a pressure transducer of type EMT 94, supplied by Elema-Schonander AB, Solna, Sweden. The pressure sensitive apparatus 90 supplies a pressure responsive signal through a line 91 to a meter 92 of the Parker type, Two Set Point Control ER 35, supplied by Parker Instrument Corporation, Samford, Conn., U.S.A.

This meter is arranged to supply a signal at a preset lower pressure limit through a line 93 connecting the meter to multivibrator 80 and at a preset upper pressure limit through a line 94 connecting the meter to multivibrator 82. In line 94 there is provided a gate 95 this gate 95 being assumed to be permanently in its open position for the time being. When the signal supplied by apparatus 90 to meter 92 through line 91 is of a value lying between the upper and lower limits set in meter 92 no signals are supplied through lines 93 and 94 to multivibrators 80 and 82, respectively, and the lung ventilator operates as described above. However, if meter 92 indicates that the lower pressure limit is underpassed a pulse is supplied through line 93 to multivibrator 80 trigging said multivibrator to immediately return to its inspiration position in order to start an inspiration period. If meter 92 indicates that the upper pressure limit is exceeded a pulse is supplied through line 94 to multivibrator 82 triggering said multivibrator to immediately return to its stable position starting the pause set by multivibrator 85 as explained above. This means that the expiration period is extended to the same degree as the inspiration period is shortened.

If the pressure in the respiration system sinks under the lower pressure limit set in meter 92 the expiration period is interrupted and an inspiration period of normal length is started. This enables the flow regulator to be adjusted so as to prevent the pressure in the pipes 62 and 65 to be lower than e.g. +7 cm H O such as during surgical operations in the thorax. Thereby, the lungs are prevented from being completely empty and to collapsing. In connection with respiratory treatment of other type when the patient is able to breath spontaneously the same system may be utilized for controlling the lung ventilator by the activity of the patient. In that case the lung ventilator is adjusted so as to supply to the patient a predetermined gas volume at a frequency securing the ventilation necessary for the patient when resting. The lower pressure limite is adjusted to start the inspiration period at a pressure in the respiration system of 0,5 cm H O. If the patient wishes to increase the ventilation, he starts the spontaneous inspiration before the inspiration phase is started by the lung ventilator. The pressure in the respiration system thereby will sink to a pressure under 0,5 cm H 0 and the lung ventilator will start the inspiration period. In this case the expiration period is shortened, the respiration frequency and the ventilation being intensified.

Optical or sound indicating apparatus may be connected to meter 92 to indicate pressures over and under the upper and lower pressure limits, respectively, established by said meter.

The breathing ofa human being is not a train of identical respiration cycles even if the breathing takes place under constant conditions. In the breathing are incorporated deep breaths known as sighs at different intervals. The lung ventilator described herein may incorporate means ascribing a sigh feature to the lung ventilator. Referring to FIGQ19 such means incorporates an astable multivibrator 100 the output of which is connected through a line 101 to a counter 102 and through a line 103 to gate 105. Multivibrator 100 supplies a pulse at preset intervals for example every fifth minute through lines 101 and 103 to counter 102 and gate 105, respectively. This pulse conditions counter 102 for counting pulses supplied thereto from the trigger output of multivibrator 82 through a line 10 4 and closes gate 95. When the next trigger pulse is supplied by mul-' tivibrator 82 this pulse is counted by counter 102 and simultaneously a trigger pulse is supplied by counter 102 through a line 105 and through line 93 to multivibrator to immediately start a new inspiration period without an intervening pause and expiration period. Counter 102 is arranged to be deactivated after a predetermined number of pulses supplied by multivibrator 82 through line 104, e.g. three pulses, and thus four inspirations are performed integrally during an extended inspiration period. When counter 102 is deactivated the normal respiration cycle is continued until next pulse is supplied by multivibrator through lines 101 and 103.

As shown in FIG. 9 a subpressure source 71 may be connected to tube 65 of the respiration system to facilitate the emptying of the lungs. In that case meter 92 may be used to start an inspiration period when the pressure in the respiration system underpasses a preset value, e.g. 10 cm H O.

It should be noted that the lung ventilator according to the invention advantageously may be adapted for autoclave treatment contrary to most prior art lung ventilators. A further important advantage of the lung ventilator according to the invention is that a sigh feature may be incorporated therein in a simple manner as described above.

We claim:

1. A lung ventilator comprising means for supplying respiratory gas, inspiration means including an inspiration line connectible to the patient, expiration means including an expiration line connectible to the patient, first means in at least one of said lines for measuring the instantaneous magnitude of at least one gas flow parameter from the group consisting of rate-of-flow and pressure, second means providing a signal proportional to said instantaneous magnitude of said flow parameter, means for providing a reference signal defining a desired flow pattern for said respiratory gas, third means for comparing the signal from said second means with the signal from said reference signal means for generating a signal proportional to the magnitude and sense of the difference therebetween, and fourth means actuated by said difference signal for automatically modulating said flow of respiratory gas, thereby to maintain a desired pattern of flow of the respiratory gas independently of the pressure of the respiratory gas furnished by said supply means and independently of the variable flow resistance and variable patient resistance.

2. A lung ventilator as defined in claim 1 wherein said first means comprises a flow meter unit, said third signal means and said flow meter unit, and said fourth means comprises a flow regulator unit for adjusting the flow of respiratory gas in response to said error signal.

3. A lung ventilator as defined in claim 2 comprising means for zeroing said flow meter unit,'means in said flow regulator unit for indicating a closed position of said unit, and means for energizing said means for zeroing said flow meter unit in response to an indication from said indicator unit that said'flow regulator unit is closed and that the flow consequently is zero.

4. A lung ventilator as defined in claim 3 wherein said zeroing means comprises an electrical capacitor, a field effect switch and said indicating means of said flow regulator unit.

- 5. A lung ventilator as defined in claim 2 wherein said flow meter unit comprising means defining two parallel passages therein, a fine-mesh netting disposed in one of said passages, a disc disposed in the other passage carried at one end by a rigid thread, a pressure sensitive member contacting said rigid thread, said pressure sensitive member being constituted by a thin silicon plate, a resistor mounted at each side of said silicon plate, said resistors being connected in a bridge connection for generating a signal in response to deformation of said silicon plate which is deformed by said rigid thread in response to the force exerted on said disc by the flow of respiration gas through said passages of said flow meter unit. I

6.'A lung ventilator as defined in claim 1 wherein said first means comprises a flow meter unit and which further includes an integrator, means applying said reference signal to said integrator, means applying the signal from said flow meter unit to said integrator, means in said integrator for integrating the deviation between the said signals applied thereto to produce a deviation signal, and means applying said deviation signal to said third means which produces said difference signal in such sense as to reduce said difference signal to zero.

7. A lung ventilatoras defined in claim 1 wherein the parameter of the respiratory gas measured is its rate-of- .flow, and which further includes a servo valve, said servo valve comprising a step motor, a cam mounted on the rotatable shaft of said step motor, a cam follower, and means including a lever actuated by said cam follower for engaging and varying the gas flow rate through a flexible hose constituting at least a section of said inspiration and expiration lines in accordance with the operation of said stepmotor. I

8. Alung ventilator as defined in claim 1 wherein the parameter of the respiratory gas measured is its rate-offlow and wherein said means providing a reference signal defining a desired flow pattern for said respiratory gas comprises a clock pulse generator wherein a predetermined number of the pulses produced thereby is definitive of one breathing cycle, a first counter receiving clock pulses from 'said generator for determining the duration of the inspiration phase of the breathing cycle in response-to a preset number of said clock pulses, means for generating a signal from said first counter in response to theduration of said inspiration phase, a second counter receiving clock pulses from said generator for determining the duration of the pause phase of said breathing cycle by counting a preset number of clock pulses corresponding to the duration of said pause time, anda logic circuit for generating an expiration signal-when sensing that there is neither inspiration phase-nor pause phase.

9. A lung ventilator as defined in claim 8 wherein said means providing a reference signal defining a desired signal flow pattern for said respiratory gas further includes a signal generator receiving signals from said clock pulse generator and from said signal generating means-of said first counter, said signal generator comprising means for generating a voltage inversely proportional to the length of the inspiration phase of a respiratory cycle, means in said signal generator for integrating the clock pulses such that the area of the reference signal is independent of the relation between the length of the inspiration phase and the length of the entire respiratory cycle and that the area of the reference signal during the inspiration phase will vary with the clock pulse frequency so that the area of all reference signals will be constant during a predetermined period of time.

10. A lung ventilator as defined in claim 9 and which further includes means for generating a desired output signal form.

11. A lung ventilator as defined in claim 9 and which comprises a further counter connected to said clock pulse generator for counting the frequency cycles of said generator, each frequency cycle corresponding to one breathing cycle, and means for decreasing the pulse frequency of said clock pulse generator after counting a preselected number of breathing cycles during the inspiration phase of the following breathing cycle thereby to simulate a sigh function.

12. A lung ventilator comprising means for supplying respiratory gas, inspiration means including an inspiration line connectible to the patient, expiration means including an expiration line connectible to the patient, first means in at least one of said lines for measuring the instantaneous magnitude of the rate-of-flow of said gas, second means providing a signal proportional to said measured gas flow rate, means generating areference signal defining a desired flow pattern for the respiratory gas, third means for comparing the signal from said second means with the signal from said reference signal means for generating a signal proportional to the magnitude and sense of the difference therebetween, fourth means actuated by said difference signal for automatically modulating the gas flow rate, a monitoring unit for monitoringthe magnitude of at least one other variable parameter of the gas in addition to its rate-of-flow for generating a signal proportional to said other monitored variable, means for indicating said last signal, and means for applying said last signal to said reference signal means to modify the signal produced therefrom.

13. A lung ventilator as. defined in claim 12 wherein said monitoring unit comprisesa mean'value flow rate integrator, means applying the signal produced by said second means indicative of the instantaneous gas flow rate in said expiration line to said integrator, means applying cloclr pulses from a clock pulse generator of said reference signal: generating means to said integrator, means for prolonging said clock pulses, means for energizing said integrator during the'duration of the prolonged clock pulses and, means for continuously generating an output signal from said integrator proportional to the integrated value of said signal produced by said second means during said prolonged clock pulses.

14. A lung ventilator comprising means for supplying respiratory gas, inspiration means including an inspiration line connectible to the patient, expiration means including an expiration line connectible to the patient,

therebetween, and fourth means actuated by said difference signal for modulating said flow of respiratory gas, thereby to maintain a desired pattern of flow of the respiratory gas independently of the pressure of the respiratory gas furnished by said supply means and independently of the variable flow resistance and variable patient resistance. 

1. A lung ventilator comprising means for supplying respiratory gas, inspiration means including an inspiration line connectible to the patient, expiration means including an expiration line connectible to the patient, first means in at least one of said lines for measuring the instantaneous magnitude of at least one gas flow parameter from the group consisting of rate-of-flow and pressure, second means providing a signal proportional to said instantaneous magnitude of said flow parameter, means for providing a reference signal defining a desired flow pattern for said respiratory gas, third means for comparing the signal from said second means with the signal from said reference signal means for generating a signal proportional to the magnitude and sense of the difference therebetween, and fourth means actuated by said difference signal for automatically modulating said flow of respiratory gas, thereby to maintain a desired pattern of flow of the respiratory gas independently of the pressure of the respiratory gas furnished by said supply means and independently of the variable flow resistance and variable patient resistance.
 2. A lung ventilator as defined in claim 1 wherein said first means comprises a flow meter unit, said third means comprises an error signal calculator receiving signals from said reference signal means and from said flow meter unit for generating an error signal defining the difference between the signal from said reference signal means and said flow meter unit, and said fourth means comprises a flow regulator unit for adjusting the flow of respiratory gas in response to said error signal.
 3. A lung ventilator as defined in claim 2 comprising means for zeroing said flow meter unit, means in said flow regulator unit for indicating a closed position of said unit, and means for energizing said means for zeroing said flow meter unit in response to an indication from said indicator unit that said flow regulator unit is closed and that the flow consequently is zero.
 4. A lung ventilator as defined in claim 3 wherein said zeroing means comprises an electrical capacitor, a field effect switch and said indicating means of said flow regulator unit.
 5. A lung ventilator as defined in claim 2 wherein said flow meter unit comprising means Defining two parallel passages therein, a fine-mesh netting disposed in one of said passages, a disc disposed in the other passage carried at one end by a rigid thread, a pressure sensitive member contacting said rigid thread, said pressure sensitive member being constituted by a thin silicon plate, a resistor mounted at each side of said silicon plate, said resistors being connected in a bridge connection for generating a signal in response to deformation of said silicon plate which is deformed by said rigid thread in response to the force exerted on said disc by the flow of respiration gas through said passages of said flow meter unit.
 6. A lung ventilator as defined in claim 1 wherein said first means comprises a flow meter unit and which further includes an integrator, means applying said reference signal to said integrator, means applying the signal from said flow meter unit to said integrator, means in said integrator for integrating the deviation between the said signals applied thereto to produce a deviation signal, and means applying said deviation signal to said third means which produces said difference signal in such sense as to reduce said difference signal to zero.
 7. A lung ventilator as defined in claim 1 wherein the parameter of the respiratory gas measured is its rate-of-flow, and which further includes a servo valve, said servo valve comprising a step motor, a cam mounted on the rotatable shaft of said step motor, a cam follower, and means including a lever actuated by said cam follower for engaging and varying the gas flow rate through a flexible hose constituting at least a section of said inspiration and expiration lines in accordance with the operation of said step motor.
 8. A lung ventilator as defined in claim 1 wherein the parameter of the respiratory gas measured is its rate-of-flow and wherein said means providing a reference signal defining a desired flow pattern for said respiratory gas comprises a clock pulse generator wherein a predetermined number of the pulses produced thereby is definitive of one breathing cycle, a first counter receiving clock pulses from said generator for determining the duration of the inspiration phase of the breathing cycle in response to a preset number of said clock pulses, means for generating a signal from said first counter in response to the duration of said inspiration phase, a second counter receiving clock pulses from said generator for determining the duration of the pause phase of said breathing cycle by counting a preset number of clock pulses corresponding to the duration of said pause time, and a logic circuit for generating an expiration signal when sensing that there is neither inspiration phase nor pause phase.
 9. A lung ventilator as defined in claim 8 wherein said means providing a reference signal defining a desired signal flow pattern for said respiratory gas further includes a signal generator receiving signals from said clock pulse generator and from said signal generating means of said first counter, said signal generator comprising means for generating a voltage inversely proportional to the length of the inspiration phase of a respiratory cycle, means in said signal generator for integrating the clock pulses such that the area of the reference signal is independent of the relation between the length of the inspiration phase and the length of the entire respiratory cycle and that the area of the reference signal during the inspiration phase will vary with the clock pulse frequency so that the area of all reference signals will be constant during a predetermined period of time.
 10. A lung ventilator as defined in claim 9 and which further includes means for generating a desired output signal form.
 11. A lung ventilator as defined in claim 9 and which comprises a further counter connected to said clock pulse generator for counting the frequency cycles of said generator, each frequency cycle corresponding to one breathing cycle, and means for decreasing the pulse frequency of saId clock pulse generator after counting a preselected number of breathing cycles during the inspiration phase of the following breathing cycle thereby to simulate a sigh function.
 12. A lung ventilator comprising means for supplying respiratory gas, inspiration means including an inspiration line connectible to the patient, expiration means including an expiration line connectible to the patient, first means in at least one of said lines for measuring the instantaneous magnitude of the rate-of-flow of said gas, second means providing a signal proportional to said measured gas flow rate, means generating a reference signal defining a desired flow pattern for the respiratory gas, third means for comparing the signal from said second means with the signal from said reference signal means for generating a signal proportional to the magnitude and sense of the difference therebetween, fourth means actuated by said difference signal for automatically modulating the gas flow rate, a monitoring unit for monitoring the magnitude of at least one other variable parameter of the gas in addition to its rate-of-flow for generating a signal proportional to said other monitored variable, means for indicating said last signal, and means for applying said last signal to said reference signal means to modify the signal produced therefrom.
 13. A lung ventilator as defined in claim 12 wherein said monitoring unit comprises a mean value flow rate integrator, means applying the signal produced by said second means indicative of the instantaneous gas flow rate in said expiration line to said integrator, means applying clock pulses from a clock pulse generator of said reference signal generating means to said integrator, means for prolonging said clock pulses, means for energizing said integrator during the duration of the prolonged clock pulses and means for continuously generating an output signal from said integrator proportional to the integrated value of said signal produced by said second means during said prolonged clock pulses.
 14. A lung ventilator comprising means for supplying respiratory gas, inspiration means including an inspiration line connectible to the patient, expiration means including an expiration line connectible to the patient, first means in at least one of said lines for measuring the instantaneous magnitude of the product of the gas pressure and of the gas flow rate, second means providing a signal proportional to said product, means providing a reference signal defining a desired flow pattern for said respiratory gas, third means for comparing the signal from said second means with the signal from said reference signal means for generating a signal proportional to the magnitude and sense of the difference therebetween, and fourth means actuated by said difference signal for modulating said flow of respiratory gas, thereby to maintain a desired pattern of flow of the respiratory gas independently of the pressure of the respiratory gas furnished by said supply means and independently of the variable flow resistance and variable patient resistance. 